1. Field of the Invention
The present invention relates to a control device for an automatic transmission, and more particularly to a technique for controlling a line pressure solenoid of an automatic transmission in a stage from engine start up to vehicle running.
2. Description of the Prior Art
The line pressure of an automatic transmission while a vehicle is running is adjusted according to the vehicle speed or throttle opening by an automatic transmission controller. The line pressure increases when the throttle opening increases.
The automatic transmission controller increases or decreases an ON/OFF duty ratio of a line pressure solenoid so as to adjust the line pressure. Specifically, an over-excitation, holding magnetization and OFF cycle is repeated with a fixed period, and the relative proportions of ON and OFF time are preset by a predetermined control amount so as to guide the line pressure to a target value.
As the line pressure is not needed when the vehicle stops and the engine stops, the automatic transmission controller maintains the holding magnetization of the line pressure solenoid so as to stop ON/OFF operation and prevent the solenoid from emitting noise. The line pressure solenoid is then in a stationary ON state wherein a control for running condition of the vehicle (ordinary control, duty ratio control) is disengaged, and the line pressure falls to its lowest level.
Japanese Patent Laid-open Publication No. 3-9169 discloses a form of control wherein, actuation of the line pressure solenoid is stopped under conditions where the line pressure is no longer required in the automatic transmission, so that wear of the line pressure solenoid and noise from the same are suppressed. In this case, the line pressure solenoid is placed in a stationary OFF state.
When over-excitation of duty ratio-controlled line pressure solenoid is continued, a large current flows and the coil lifetime decreases. Therefore when the line pressure solenoid is brought into a stationary ON state, a high over-excitation voltage is applied for a very short time after the key switch is switched ON so as to switch the solenoid ON, and the solenoid is subsequently maintained in the ON state at a low holding magnetization voltage.
However, when the engine is started in this stationary ON state, the battery voltage temporarily decreases due to a large current taken by the cell motor. The drive voltage of the line pressure solenoid may then also fall so that the solenoid switches OFF.
Although the low holding magnetization voltage is sufficient to keep the solenoid ON in opposition to the counterforce of a return spring in the solenoid, but it is not sufficient to compress the spring and bring the solenoid back to ON position from its OFF position.
Consequently when the cell motor drive stops, even if the battery voltage is restored and the drive voltage of the line pressure solenoid is restored to its normal level of holding magnetization, the line pressure solenoid does not return to its normal ON state and remains OFF.
It has been proposed to detect the battery voltage and to apply intermittent over-excitation for a period in which the battery voltage is lower than a predetermined value, so as to restore the line pressure solenoid from OFF to ON state. In this case however, it is necessary to provide a circuit for detecting battery voltage in the automatic transmission controller.